Single phase AC motors comprise a main winding and an auxiliary or start winding used for starting the motor. Various means have been employed to disconnect the start winding when the motor reaches a selected speed, such as centrifugal mechanisms, current relays and, more recently, solid state electronics circuits that respond to time or various motor operating parameters.
With respect to such means electro-mechanical devices have a disadvantage of being subject to wear over the years adversely affecting their reliability and solid state electronics circuits are relatively expensive and their dependability is open to question.
In U.S. Pat. No. 3,600,656, which issued in 1971, it is noted that it was known to connect a PTC resistor in series with the start winding, the PTC resistor being heated by the current that flows through the start winding causing the resistance of the PTC resistor to rise and reduce the current flowing through the start winding to a low level in a prescribed period of time. However, the continued current flow, though relatively of a low level, is inefficient and represents a loss of power. Further, the PTC resistor is continuously heated so that it presents a problem in cases where restarting of the motor is desired since the PTC resistor would still be in a blocking mode of operation.
U.S. Pat. No. 7,061,204 shows a PTC resistor serially connected to a start winding and to a triac for de-energizing the start winding. In this apparatus, the gate terminal of the triac is connected to a control circuit that includes a current detecting circuit. While the motor starter is effective to de-energize the PTC resistor and provides low power by essentially eliminating the start winding following start-up, this starter, which employs a current transformer, is relatively expensive.
It is also known to use a reed switch in the gate circuit of a triac that is serially connected to a start winding in order to effectively cut out the start winding following start-up. In U.S. Pat. No. 4,307,327, a triac is connected to a start winding and a reed switch is connected to the gate terminal of the triac. The reed switch has a sense coil that receives the combined start and main winding current of the motor. As the motor approaches synchronous speed with its concomitant lower main winding current the contacts of the reed switch open and turn off the triac thereby eliminating current flow through the start winding. However, this starter is designed to be used with capacitor start motors rather than with resistance start motors typically used for refrigerator compressors.